Well, there's a bit of speculation in this thread. In Canadian County, in general, a lot is often averages about 40 acres. It occurs often because of rivers such as the Canadian River. Interestinging, landowners do NOT own land on the River, and mineral owners do NOT own minerals under the river. That produces some sections which have a little less than 640 acres in total. There are also some sections with MORE than 640 acres. I happen to know, since I own farms there, that the Canadian River does not run through your place....but never mind, there is something irregular about that section.
Sounds like your grandfather owned a 5/8 mineral interest in approximately 160 acres, and your mother inherited a third.
Linn Energy seems to have several vertical wells in the section with 640 acre spacing. The leases seem recent enough that you probably got 3/16 royalty, but you would need to check the oil and gas lease that your grandparents signed. If you could pass on the "decimal interest" from her Linn Energy checkstub, I could confirm the number of acres involved.
In recent history, Marathon completed a Woodford well in this section (with 640 acre spacing) in May 2009. This was one of the early Woodford wells, and had only a half mile lateral. She should be getting a check from Marathon, The well is the Calhoon 1-7H, and it came in producing a 1001mcf/day in gas with pressure at 1400# on an 18/64th choke. Recent production shows that it has made about 350 mcf/day. This is not bad for one of the early wells. It makes in total about $900 per day, which is only a little different than a million dollars a day (!). On a 3/16 lease, based on the ownership acreages you passed on, that would be about $250/month. These are just rough estimates. In addition, there would be income from the "liquids" produced, and that can be significant.
However, the future is bright. Devon is doing a lot of "infill drilling" only a couple of miles to the north. If you drive out there, you will see many rigs lined up in a row. What this means generally is that they will be drilling "up to" 8 additional wells per section here in the core of the Woodford Shale. I did not look at the particular order for your section, but that generally is what is going on. In addition, you can bet they will be doing at least a full mile lateral with a much improved frac job now, and so your expectations should be higher.
I own mineral mineral rights in the section adjoing your mother's section, and in a few other sections where I have farms a little farther north. If you have any more questions, let me know. I have tried to keep up with the activity out there pretty carefully.